In a semiconductor manufacturing process, a bare wafer has to go through hundred of processes to form integrated circuits to function as desired. The processes include coating photoresist, photo developing, depositing and etching. These processes are targeted on the front side of the wafer. Even planarization and cleaning processes focus only the front side. The wafer backside has rarely been protected. However in the processes set forth above, wafer backside also has chance being eroded by chemicals, contaminated by photoresist residuals or other exhaust particles. All this has negative impact on wafer quality and may reduce process yield.
There is a prior art disclosed in U.S. Pat. No. 5,689,749 for protecting wafer backside. FIGS. 1 and 2 show its general structure. It includes a motor 33, a chuck 21 and an annular seat 26.
The motor 33 is mounted on an elevator mechanism 34 and has an output shaft 14 to provide a rotation power source. The elevator mechanism 34 may drive the motor 33 up or down.
The chuck 21 has a vacuum suction means (not shown in the figures) in the center for sucking and holding the backside of the wafer W. The annular seat 26 is located below the wafer W and surrounds the chuck 21. On the annular seat 26, there is mounted a water guard ring 42 which has a top rim adjacent the wafer backside. The top rim has a specially formed inner ring 40b and an outer ring 40a. In the annular seat 26, there are also a pair of nozzles 32 symmetrically located. Outside the annular seat 26, there is an annular chamber wall 20. The nozzles 32 have their ejection openings facing the water guard ring 42 and may eject protection liquid (such as D.I. water). The chamber wall 20 may shield the process chamber from the outside environment.
In the process chamber above the wafer W, there are nozzle heads 23 and 31 for providing reaction agent needed for semiconductor manufacturing process.
When a robot arm (not shown in the figures) moves the wafer W into the process chamber, the elevator mechanism 34 drives the chuck 21 to suck and hold the wafer W and moves the wafer W to an initial position. The motor 33 and the output shaft 14 then drive the chuck 21 and the wafer W to rotate. The nozzle heads 23 and 31 eject reaction agent on the wafer W surface. Because of centrifugal force, the reaction agent on the water surface will be spun out. Some of the reaction agent will hit the chamber wall 20 and rebound on wafer backside.
In order to prevent the backside of the wafer W from contamination by the reaction agent, the nozzles 32 will eject protection liquid to the water guard ring 42. As the top end of the water guard ring 26 adjacent the wafer backside, when the water guard ring 26 fills with the protection liquid and spills over, the rotating wafer W will pickup some protection liquid to form a protection liquid film around the inner ring 40b (or outer ring 40a). This liquid film may protect the wafer backside from outside contamination.
However the prior art set forth above has complex components and forms. They are difficult to produce and assemble. The production cost is higher. It also does not provide a closed process environment and is prone to contamination. Furthermore, ejection of protection liquid from the nozzles 32 requires high pressure and often results in return splashing of the protection liquid. This will weaken the suction force of the chuck 21 to the wafer W. The strong liquid impact could even break the wafer W.
In the semiconductor industry, there are some other techniques have been developed to address this issue, using gas film (such as nitrogen gas film) to fill the gap between the wafer backside and the annular seat is one of such techniques. However gas film is not effective to provide a closed process environment. There is still room for improvement regarding the issue of wafer backside protection.